1. FIELD OF THE INVENTION
This invention generally relates to sealing the space between opposing spaced surfaces or ledges and particularly to a new and improved sealing arrangement for implementing a temporary seal to provide a floor for supporting a liquid. More specifically, the present invention is directed to a method for establishing a seal between a pair of spaced generally planar annular surfaces and especially to forming a leak-proof demountable floor for a refueling canal for a nuclear reactor. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. DESCRIPTION OF THE PRIOR ART
The present invention is particularly well suited for use in nuclear reactor power plants for establishing a temporary seal between the reactor pressure vessel and the inner wall of the vessel containment structure. The refueling process for pressurized water reactors is ordinarily performed under approximately 25 feet of water in a refueling canal disposed above the main body of the reactor vessel. During the refueling process, the reactor vessel cavity in the containment structure under the canal must remain dry to prevent contamination of the vessel or equipment located in the cavity about the exterior of the vessel. During normal power operation governmental and licensing regulations require that no liquid remain in the refueling canal so that an emergency escape path is formed. Consequently, the sealing arrangement which isolates the bottom of the refueling canal from the lower cavity during the refueling process can not remain in place during normal reactor operation.
The reactor vessels of conventional pressurized water reactors have a circumferential flange which extends transversely outwardly from the vessel wall. The vessel is mounted in a cavity formed by a massive containment structure. A ledge is formed in the containment structure in spaced relationship to the vessel flange. The upper surfaces of the flange and ledge are substantially co-planar. Conventional arrangements for temporarily forming the floor of the refueling canal involve the use of massive steel ring plates which may have outside diameters as large as 25 feet and widths varying from one to three feet. The ring plates are suspended from the flange and the ledge. Compression seals carried by the ring plate seal against the flange and the ledge. The ring plate may be bolted to compress the seals and form a liquid impervious seal.
One of the principal deficiencies of the foregoing conventional sealing arrangement is the requirement that the lower surface of the ring plate be precision machined during the fabrication process in order to provide a sealing surface for the compressive sealing engagement throughout the circumferential extent of the plate/seal interface. The necessity of precise machining of the ring plate essentially precludes assembly of the plate in the field. Consequently, the massive size of the conventional ring plate generally requires that the containment building be erected subsequent to the arrival of the plate at the reactor site. In addition, thermal expansion and structural shiftings of the reactor structures over time tend to adversely modify the sealing characteristics of the ring plates thereby requiring additional compensating arrangements to insure adequate sealing integrity.
More recent sealing arrangements involve substituting an inflatable seal for the compression seals and bolts to compensate for imperfections in the ring plate sealing surface and the shifting of the flange and/or ledge structures. The inflatable seals are energizable for expansion between the flange and the plate to provide a high degree of sealing integrity. In U.S. Pat. No. 4,214,760, entitled "Seal For Adjacent Plates", an arrangement for temporarily sealing the space between two substantially parallel adjacent plates employs a single flexible partially inflatable seal located between the plates to effect a primary and a secondary sealing interface along the upper and lower edges of the plates. The seal has a wedge-shaped upper portion, a tubular lower portion and a substantially solid intermediate shank. The tubular end of the seal is inflated so that it expands into contact with the lower edges of the adjacent plates thereby drawing the wedge-shaped upper portion of the seal toward the tubular portion to form a primary sealing interface between the wedge-shaped portion and the upper edges of the plates. The contact of the inflated tubular portion with the lower edges of the plates forms the secondary seal interface. The seal is flexible and thus can conform to rough surfaces such as may be present on reactor pressure vessel flanges, containment structures and ring plates.
Any sealing arrangement which employs inflatable seals without a reserve or backup seal mechanism is susceptible to failure in the event of failure of an inflatable member.